Integrated lithostratigraphic and geochronologic studies of Umm Akhtar Playa, a newly discovered dry lake located at ≈ N 22° 36' E 30° 18' in s. Egypt, indicate the accumulation and rapid desiccation of a sizeable (>65 km²) standing water body during the Late Quaternary. An optical date of 15,120 cal yr on basal aeolian sediments indicates that the deflation basin was mantled with aeolian sands during the Late Pleistocene. Overlying Holocene fluvio-lacustrine muds and gravels attests to the former availability of ponded runoff waters at this now-hyperarid locality. Playa waters were deep and persistent enough to create beach berms composed of well-rounded pebbles, and to sustain cultural activities of Neolithic pastoralists. Twenty-two radiocarbon dates bracket 'wet' phases and concurrent prehistoric human occupation from ≈ 8915-8580 ¹⁴C yr BP (9925-9544 cal yr BP) and ≈ 7105-5955 BP (7878-6799 cal yr BP). Increasing amounts of sand were incorporated into the playa after 7500 ¹⁴C yr BP; two optical dates constrain the timing of enhanced aeolian activity from 6720-6250 cal yr. These optical data comprise the only existing absolute ages for the emplacement of aeolian sediments during the rapid transition from wet (playa) to dry (sands) conditions in the Holocene. Playa desiccation after 5955 ¹⁴C yr BP (6799 cal yr BP) was marked by cultural abandonment, evaporite precipitation, dune migration, and deflation. The AMS dates from Umm Akhtar Playa fall in the same range as published radiocarbon data from contemporary sites and playas in Egypt and northern Sudan; the main period of enhanced surface water storage from 8100-6000 BP is linked to the intensified Afro-Asian monsoon forced by cyclical astronomical variations. The record from the Arba'in Desert of Egypt and northern Sudan lags the Northern Hemisphere seasonal insolation maximum (centered at 10,400 BP) and the greatest frequency of African lake highstands, (9500-8500 BP) and may result from biogeophysical feedback mechanisms involved with the 'recycling' of water within the continental interior. Sub-millenial variability apparent in the composite record from Egypt likely reflects the influence of atmospheric-oceanic dynamics throughout the Holocene.

Topographic transects corning from four different Digital Elevation Models (DEMs) which collectively span different ranges of scales from 20000 km to 0.5 m are analyzed. Power spectra, trace moments and structure functions are used to show that continents and oceans have the same moment scaling function K(q) but different (scale by scale) nonconservation H and also that the Earth's topography is multiscaling from planetary scales down to a few meters. The results also suggest that topography can be described statistically at all scales by a global K(q). The form of K(q) shows that the statistics of topography are close to those predicted by universal multifractals. It seems that the multiscaling of topography is broken because of the presence of trees on the DEM: they introduce a characteristic length in the vertical that is approximately 9 m.

Crop production in Brazil has changed significantly over the last decade. New crops are being cultivated to satisfy the world's growing demand for Brazilian export products —a demand that has caused substantial changes in land use and cover, mainly characterized by the increase in large-scale mechanization of agriculture, deforestation, and intensification of agricultural land use. Brazil currently provides crop production information at the municipality level. This information was analyzed using Geographic Information Systems (GIS) to examine changes in the spatial distribution of the production of various crops and livestock in Brazil for 1990-2006. In addition, to better understand the relationship between agricultural expansion and deforestation, spatial data on agricultural expansion and deforestation over the Legal Amazon were statistically analyzed for 2000-2006. The results indicate that changes in the spatial patterns of crops have indeed taken place in central and northeastern Brazil as well as in the southern Amazon region. The areas to crops such as soybean and sugarcane expanded, surpassing the total area planted to domestic food crops, which, in turn, recorded a significant decrease in area. This crop expansion has exerted pressure on other crops and livestock, pushing them further into the Amazon forest region during 1990-2006. In the same period, pasture was the predominant land use in the Legal Amazon; however, results indicate that the area planted to soybean increased whereas the area under pasture decreased. Statistical analyses revealed that, in those areas with over 50% forest, deforestation was strongly related to agricultural expansion. Defore / La production agricole du Brésil a changé significativement durant la dernière décennie. De nouvelles cultures ont été adoptées afin de répondre à la croissance de la demande mondiale pour des produits d'exportation brésiliens – une demande qui a occasionné des changements substantiels au niveau de l'utilisation et de la couverture du sol, principalement caractérisés par l'accroissement à large échelle de la mécanisation de l'agriculture, de la déforestation et de l'intensification de l'agriculture. Le Brésil met à disposition de l'information concernant la production agricole au niveau municipal. Cette information a été analysée par le biais d'un Système d'Information Géographique (SIG) afin d'étudier les changements dans la distribution spatiale de la production de différentes cultures et d'élevage au Brésil de 1990 à 2006. De plus, afin de mieux comprendre la relation entre l'expansion agricole et la déforestation, des données spatiales ont été analysée statistiquement pour l'Amazone Légale pour une période allant de 2000 à 2006. Les résultats indiquent que des changements dans les patrons spatiaux ont en effet pris place au centre et au nord-est du Brésil ainsi qu'au sud de la région amazonienne. Les zones prévues pour cultiver le soja et la canne-à-sucre ont augmenté, surpassant même les surfaces semées pour des cultures vivrières qui ont par ailleurs enregistrées une diminution significative. L'extension de ces cultures a exercé une pression sur les autres cultures et sur les élevages bovins, les poussant à l'intérieur de la forêt amazonienne durant la période 1990-2006. Au cours de la même période, alors q

The abrasive effects on periphyton biomass of saltating sand, that was transported over a stable gravel-cobble riverbed during summer flow events (spates), was studied on 35 riffles of an oligo-mesotrophic river in Quebec. A periphyton saltation abrasion model (PSAM) was developed from an existing bedrock erosion mechanistic model (Sklar and Dietrich 2004). The empirical PSAM models explained 56-59% of the variance in post-spate biomass on individual host-rocks following a low-magnitude, high-frequency spate. Periphyton biomass was curtailed on riffle rocks when sand transport rates exceeded 64-180 g m-1event-1 during spates. This perturbation threshold was confirmed by an in situ experiment that documented periphyton losses resulting from increasing rates of sand transport. Low profile substrates (2 cm) were scoured more thoroughly than high protruding substrates (6 cm), confirming the PSAM hypothesis that refuge potential increases with elevation above the stream bed relative to the mean sand saltation height. An analysis of spatial patterns of periphyton biomass across riffles confirmed the tendency for a spate-related refuge habitat between the edge of the varial zone (zone of frequent wetting and drying) and the thalweg (deepest point) of the channel. A hierarchical model (HPPM) that simulates how periphyton biomass is regulated by the flow of water and the flux of sand down a sedimentary link (segment of river along which grainsize and slope decreases systematically downstream) was developed and validated. Sedimentary links contained 3 channel types (hydrogeomorphic reaches, HGRs), each possessing significantly different morphological traits, ranging from straight-steep, cobble-bed reaches (High-HGR) at the upstream end, to low-gradient, sinuous gravel-bed reaches (Low-HGR) downstream. For low-magnitude spates (e.g. 0.85QMean_Annual), the most refuge (i.e. highest post spate biomass) was provided o / Les effets abrasifs du sable en saltation sur la biomasse de périphyton, lorsqu'il est transporté au-dessus d'un lit de gravier et de galets stable durant les crues estivales, ont été étudiés sur 35 riffles d'une rivière oligomésotrophique québécoise. Un modèle d'abrasion du périphyton par saltation (PSAM) a été développé à partir d'un modèle mécanistique de l'érosion du soubassement. (Sklar et Dietrich, 2004). Les modèles PSAM empiriques expliquaient 56% à 59% de la variance de la biomasse après-crues sur les roches, suite à une période de crues de basse magnitude et de haute fréquence.La biomasse du périphyton sur les roches des riffles était limitée par une vitesse de transport du sable qui excédait 64-180 g m-1 événement-1 durant une période de crues. Ce seuil de perturbation a été confirmé par une expérience in situ, où nous avons observé les pertes de périphyton causées par des vitesses croissantes de transport du sable. Le substrat à profil-bas (2 cm) a eu des pertes plus importantes de périphyton relativement au substrat saillant (6 cm), ce qui confirme l'hypothèse PSAM que le potentiel du refuge augmente avec l'élévation au-dessus du lit. Une analyse de la distribution spatiale de la biomasse de périphyton dans les riffles a confirmé qu'une zone de refuge de crue existait entre les bords de la zone variable (zone fréquemment mouillée et séchée) et le thalweg du canal (le point le plus profond). Un modèle de perturbation hiérarchique de périphyton (HPPM) a été développé pour simuler l'influence du transport de l'eau et le flux du sable sur la biomasse de périphyton à travers un lien sédimentaire (section de rivière de plusieurs km de longueur, où la dimension des roches diminue systématiquement en aval). Les liens sédimentaires sont divisés en 3 échelles de refuge : des lits de galets droits et raides (haute HGR) en amont, aux li

An essential requirement in evaluating the potential impacts of climate warming in Polar Regions, is a greater understanding of the nature and behavior of the active layer and the permafrost system. Numerous studies have documented the inherent capacity of permafrost to limit significant degradation from seasonal thawing because of the ability of overlying soil layers to dissipate influxes of heat. However, information on the degree to which these layers are able to act as buffers to permafrost degradation is limited, and thus further research on this system is needed.The research presented in this thesis in the form of two manuscripts. The first manuscript (Chapter 3) discusses the role of chemical processes in the soil weathering regime on Herschel Island. Three distinct cryostratigraphic layers were observed in the soil profiles, including; (1) the modern active-layer located immediately below the ground surface (0 – 39 cm); (2) a paleo-active (transition layer), extending from the bottom of the modern active layer to the top of an undisturbed permafrost layer (39 – 192 cm); and (3) the underlying permafrost. These layers are marked by two thaw unconformities that separate the modern active layer from the transition layer and the transition layer from the permafrost layer, respectively. The two thaw unconformities were distinguished by abrupt breaks in the cryogenic texture and peaks in the concentrations of several chemical components. The upper thaw unconformity, located at the base of the modern active layer, coincided with a doubling in the concentration of major soluble cations and a tripling in the organic matter content compared to those of the overlying soil horizons. The lower thaw unconformity is marked by the highest concentration of Al2O3 and K2O in the entire profile and the second highest concentration of Na2O. Between these two unconformities, (i.e. in the transition zone), soluble cation concentrations varied considerably, the bulk soil SiO2 concentration increased significantly and there was a noticeable decrease in the concentrations of Al2O3, Fe2O3, CaO, MgO and K2O in the depth interval of 82-107 cm. Chemical weathering indices recorded an overall decrease in the intensity of chemical weathering with depth; the soil layers immediately above the first thaw unconformity were the most weathered and the deeper permafrost layer was the least weathered.The second manuscript (Chapter 4) employs a heat conduction algorithm (Stefan equation) based on the composition and thermal properties of the soils discussed in Chapter 3, and measured active layer depth values, to evaluate heat transfer from the ground surface to the underlying permafrost. Thermal constants (the square root of the ratio of twice the thermal conductivity of the unfrozen soil to the latent heat of fusion of ice) were calculated using the De Vries method and were compared to empirical thermal constants derived from measurements of active layer depths and the corresponding thawing degree days using the Stefan equation. The theoretical constants are approximately 38% lower than those derived empirically, a difference that is likely due to the fact that the Stefan equation does not consider advective heat transfer and that the active layer depths were probably overestimated because of the method of measurement (resistance to penetration of a metal probe). Third generation Canadian Global Coupled Models (CGCM 3.1/T63) from the Canadian Centre for Climate Modelling and Analysis (CCCma), based on the IPCC SRES A2 and B1 scenarios, were used in conjunction with the theoretically and empirically derived thermal constants to predict active layer depths for 2050 and 2100. These calculations suggest that the active layer will thicken by up to 41.76 % before the end of this century. This thesis provides new insights into the important role that the active and transition layers play in controlling the response of permafrost systems to climate change. / Une condition essentielle pour évaluer les impacts potentiels du réchauffement climatique dans les régions polaires est l'acquisition d'une meilleure compréhension de la nature et du comportement de la couche active du pergélisol. La recherche présentée dans cette thèse est présentés sous la forme de deux manuscrits. Le premier manuscrit examine le rôle des processus chimiques dans le régime d'altération du sol sur l'île Herschel. Trois couches distinctes cryostratigraphiques ont été observées dans les profils de sol: (1) la couche active moderne située immédiatement sous la surface du sol (0 - 39 cm), (2) la couche paléo-active (couche de transition) s'étendant de la base de la couche moderne active à la partie supérieure de la couche de pergélisol non perturbée (39 - 192 cm) et (3) le pergélisol sous-jacent. Ces couches sont marquées par deux discordances de dégel séparant la couche active moderne de la couche de transition et la couche de transition de la couche de pergélisol, respectivement. Ces deux discordances de dégel ont été identifiées grâce à la présence de ruptures brutales dans la texture cryogénique et par des pics dans la concentration de plusieurs composants chimiques. La discordance de dégel supérieure, située à la base de la couche moderne active, coïncide avec un doublement de la concentration des principaux cations solubles et un triplement de la teneur en matière organique par rapport aux horizons sus-jacents. La discordance de dégel inférieure est marquée par la plus forte concentration en Al2O3 et en K2O de l'ensemble du profil et par la deuxième plus forte concentration en Na2O. Entre ces deux discordances, les concentrations en cations solubles varient considérablement, la concentration en SiO2 du sol brut s'accroit de façon significative et on observe une diminution notable des concentrations en Al2O3, Fe2O3, CaO, MgO et K2O dans l'intervalle de profondeur de 82-107 cm. Les indices d'altération chimique ont enregistré une diminution globale de l'intensité de l'altération chimique avec la profondeur ; les couches de sol immédiatement au-dessus de la première discordance de dégel sont les plus altérées et la couche profonde du pergélisol est la moins altérée.Le second manuscrit utilise un algorithme de conduction de la chaleur basé sur la composition et les propriétés thermiques des sols examinés au chapitre 3 et sur les mesures des valeurs de profondeur de la couche active afin d'évaluer le transfert de chaleur de la surface du sol au pergélisol sous-jacent. Des constantes thermiques (la racine carrée du rapport entre deux fois la conductivité thermique du sol non gelé et la chaleur latente de fusion de la glace) ont été calculées en utilisant la méthode de De Vries et comparées aux constantes thermiques empiriques issues des mesures de profondeur de la couche active et des degrés-jours de dégel correspondant en utilisant l'équation de Stefan. Les constantes théoriques sont environ 38% inférieures à celles obtenues empiriquement, une différence qui s'explique probablement par le fait que l'équation de Stefan ne considère pas le transfert de chaleur par advection et que les profondeurs de la couche active ont probablement été surestimées en raison de la méthode de mesure (résistance à la pénétration d'une sonde métallique). La troisième génération du modèle couplé climatique global canadien (CGCM 3.1/T63) du Centre canadien de la modélisation et de l'analyse climatique (CCmaC), basée sur les scénarios A2 et B1 du IPCC SRES, a été utilisée en conjonction avec les constantes thermiques théoriques et empiriques pour prédire les profondeurs de la couche active pour 2050 et 2100. Ces calculs suggèrent que la couche active s'épaissira de jusqu'à 41,76% avant la fin du siècle.Ce mémoire apporte un nouvel éclairage sur le rôle important que les couches actives et de transition jouent dans le contrôle de la réponse des systèmes de pergélisol au changement climatique.

Since vegetation exerts strong controls on local ecosystem processes, understanding the effects of disturbance on short-term and long-term revegetation patterns is a critical component of understanding the effects of climate change on the Arctic. Arctic landscapes underlain by massive ground ice and ice-rich permafrost are inherently unstable and often display evidence of past and present thaw subsidence. Retrogressive thaw slumps are permafrost thaw features which are progressively backwasting and tend to go through cycles of activity, resulting in dramatic changes to the landscape. The cyclic pattern of disturbance and stabilization related to these thaw slumps results in a patchy tundra landscape where there are easily identifiable geomorphic units reflecting the stage of stabilization and the time since disturbance. This research investigates natural revegetation and patterns of succession following permafrost disturbance by retrogressive thaw slumps on Herschel Island, Yukon. With future warming expected, the presence and areal extent of thaw slumps on Herschel Island and the Yukon North Slope are predicted to increase. The research objectives of this work include determining the diagnostic features of disturbed surfaces, describing the vegetation succession following thaw slump disturbance, and creating a baseline vegetation dataset for future comparison. Seven sites were chosen for the study, representing undisturbed areas in addition to 250 year old, 20 year old, and 10 year old stabilized thaw slumps. Species presence, diversity, and cover are used to describe the plant community. Percent cover vegetation data are analyzed using non-metric multidimensional scaling (NMDS), multi-response permutation procedures (MRPP), and hierarchical clustering analysis (HCA). Indicator species are identified for each age class, and the pH, organic matter content, active layer depth, and gravimetric water content of each age class' soil are presented. Results indicate that distinct vegetation communities and soil characteristics are associated with each age class, with the least differentiation occurring between the 10 and 20 year old sites. The observation that after 250 years the stabilized slump area does not support the same vegetation community as the undisturbed tundra has important implications for the complex biotic relationships of the Western Arctic. These findings suggest that if future climate warming leads to an increase in the number of retrogressive thaw slumps, the species composition of the area will be altered and certain plant species may become scarcer. / La végétation exerce des contrôles puissants sur le fonctionnement des écosystèmes. Comprendre les effets des perturbations sur les motifs de revégétalisation à court et long terme est donc un élément crucial dans la compréhension des effets des changements climatiques sur l'Arctique. Les paysages Arctiques qui reposent sur des dépôts massifs de glace souterraine et de pergélisol riche en glace sont essentiellement instables et montrent souvent des signes de subsidence dus au dégel dans le passé et dans le présent. Les glissements rétrogressifs dus au dégel sont des traits caractéristiques du retrait progressif des glaces et ont tendance à entrer dans des cycles d'activités, pouvant amener des changements drastiques au paysage. Les motifs cycliques des perturbations et stabilisations reliées à ces glissements dus au dégel donnent comme résultat un paysage de toundra inégal où les unités géomorphologiques reflétant l'étape de stabilisation et le moment de la perturbation sont facilement identifiables. Ce projet mène une enquête sur la revégétalisation naturelle et les motifs de succession suite à des perturbations dues à des glissements rétrogressifs dus au dégel sur l'Île Herschel au Yukon. Vue le réchauffement prévu dans le future, la présence et superficie des glissements dus au dégel sur l'Île Herschel et du Versant nord du Yukon sont prévus d'augmenter. Les objectifs de recherche de ce projet sont de déterminer les caractéristiques diagnostiques des surfaces perturbées, de décrire la succession de la végétation suite au glissement dû au dégel, et de créer une base de données référant à la végétation qui pourra servir lors de comparaisons futures. Sept sites ont étés choisis pour ce projet, représentant des zones non-perturbées ainsi que des zones de glissements dus au dégel stabilisés depuis 250 ans, 20 ans, et 10 ans. La présence des espèces, la diversité et la couverture ont été utilisés afin de décrire la communauté végétale. Les données sur le pourcentage de couverture végétale ont étés analysés avec le positionnement multidimensionnel non-métrique (NMDS), le multi-response permutation procedures (MRPP), et la classification ascendante hiérarchique (CAH). Les espèces indicatrices ont été identifiés selon leur classe d'âge, et pour chacune le pH, le contenu de matière organique, la profondeur de la couche active et la teneur en eau pondérale du sol sont présentés. Les résultats indiquent que les communautés végétales et caractéristiques du sol distinctes sont associées à chaque classe d'âge, la différentiation la moins importante ayant lieu à l'intervalle des sites de 10 et 20 ans. L'observation qu'après 250 ans la zone de glissement stabilisée ne supporte plus la même communauté végétale que les zones non-perturbées de la toundra a des implications importantes pour notre compréhension des relations biotiques complexes dans l'Arctique de l'ouest. Ces résultats suggèrent que si le réchauffement climatique futur mène à une augmentation des glissements rétrogressifs dus au dégel, la composition des espèces sera modifiée et certaine espèces végétales pourraient devenir plus rare.

Salmon and trout respond to river temperature fluctuations by moving into cooler areas when main-stem temperatures exceed their upper tolerance limits. In a climate warming context, these areas are increasingly recognized as an important aspect of stream habitat especially for cold water fish such as salmonids, because they can act as thermal refuges during summer months. This thesis aims firstly to examine the physical processes that underlie the formation of various types of cold water anomalies in a river setting, and secondly, to determine which anomaly types are being used as refuges by salmonids in hot weather. Two main refuge forming processes are investigated, this in two Quebec salmon rivers. These are 1) thermal plume formation from cool tributaries inflowing into warm main-stem reaches, and 2) vertical thermal stratification, with cooler water below warmer surface water. The dynamics of thermal refuges thus formed are characterized using environmental monitoring techniques, with various morphometric and physicochemical measurements describing the variability of existing conditions within these refuges. Visual fish surveys are also carried out in an event of moderate heat stress, to document the use by salmonids of cold, thermally stratified anomalies. A simple hydrodynamic modelling tool is used to determine how the local morphology and hydraulics of a confluence zone can influence the size and geometry of tributary-sourced thermal refuges; this tool is then coupled with data on future regional climate scenarios, to assess how climate change might affect a typical tributary plume refuge. Finally, select statistical methods are combined to determine the morpho-hydraulic characteristics that are associated with thermal stratification in the systems under study, as well as the habitat conditions that are preferably selected by salmonids when they are seeking refuge. / Le saumon et la truite répondent aux fluctuations de température des rivières en se déplaçant vers des zones fraîches lorsque les températures du chenal principal dépassent leurs seuils supérieurs de tolérance. Dans le contexte du réchauffement climatique, ces zones sont de plus en plus reconnues comme constituant un aspect important de l'habitat fluvial, surtout pour les poissons d'eau froide tels que les salmonidés, puisqu'elles peuvent agir comme refuges thermiques durant les mois d'été. La présente thèse a d'une part pour objectif d'examiner les processus physiques menant à la formation d'anomalies thermiques froides en rivière, et d'autre part, de déterminer quels types d'anomalies sont utilisés par les salmonidés par temps chaud. Deux principaux processus sont ciblés, et ce pour deux rivières à saumon du Québec. Ces processus sont 1) la formation de panaches thermiques issus de la confluence de tributaires froids avec le chenal principal, et 2) la stratification thermique verticale, avec eau plus froide demeurant sous la surface. Des techniques de surveillance environnementale sont employées pour caractériser la dynamique des refuges thermiques ainsi formés, et des mesures tant morphométriques que physicochimiques sont effectuées, afin de dresser un portrait de la variabilité des conditions existantes au sein de ces refuges. Des relevés visuels sont réalisés lors d'un événement de stress thermique modéré, dans le but de documenter l'utilisation par les salmonidés de refuges froids stratifiés. Un outil simple de modélisation hydrodynamique est utilisé pour déterminer comment la morphologie et l'hydraulique locale d'une zone de confluence peuvent influencer la taille et la géométrie des refuges issus de tributaires froids; cet outil est aussi couplé à des données de scénarios climatiques futurs, afin d'évaluer comment le changement climatique pourrait affecter un refuge thermique typique. Enfin, des méthodes statistiques sont combinées pour déterminer les facteurs morpho-hydrauliques qui sont associés à la présence de stratification thermique au sein des systèmes à l'étude, ainsi que les conditions de l'habitat qui sont sélectionnées de préférence par les salmonidés lorsqu'ils cherchent refuge.

<p> Northeastern and mid-Atlantic United States are understudied from the perspective of hurricane vulnerability. In an attempt to fill this gap in research, this dissertation attempted to assess the hurricane vulnerability of the northeastern and mid-Atlantic United States through the construction of a Composite Hurricane Vulnerability Index (CHVI) for 184 counties extending from Maine to Virginia. The CHVI was computed by incorporating indicators of human vulnerability and physical exposure. Human vulnerability was derived from demographic, social and economic characteristics whereas physical exposure was based on attributes of the natural and built up environments. The spatial distribution of the CHVI and its component indices were examined and analyzed to meet the research goals, which were a) to develop indices of human vulnerability, physical exposure and composite hurricane vulnerability for all counties; b) to assess vulnerability distribution in terms of population size, metropolitan status (metropolitan versus non metropolitan counties) and location (coastal versus inland counties); c) to identify the specific underlying causes of vulnerability; d) to identify the significant clusters and outliers of high vulnerability; and e) to examine overlaps between high human vulnerability and high physical exposure in the region.</p><p> Results indicated high overall vulnerability for counties that were metropolitan and / or coastal. Vulnerability was high at both ends of the population continuum. Coastal areas had high natural exposure whereas metropolitan areas had high built exposure. In large metropolitan counties, human vulnerability was influenced most strongly by economic vulnerability. In non-metropolitan and small metropolitan counties, vulnerability was an outcome of a combination of demographic, social and economic factors. Vulnerability clusters and intersections pointed towards high vulnerability in the major cities along the northeastern megalopolis, in the Hampton Roads section of Virginia and in parts of Delmarva Peninsula. </p><p> Research findings have important implications for disaster management. Evidence of relationship of population size, metropolitan status and location with vulnerability levels provides a new perspective to vulnerability assessment. Identification of high vulnerability counties can lead to effective resource allocation and emergency management and mitigation plans. Detection of dominant underlying causes of vulnerability can help develop targeted strategies for vulnerability reduction.</p>

<p> This study focuses on the western Gale Hills located in the western portion of the Lake Mead domain in southern Nevada. The western Gale Hills preserve a record of the Miocene sedimentation and deformation related to the breakup of the hanging walls of the South Virgin-White Hills detachment fault and the Lime Ridge oblique, strike-slip fault of the Lake Mead fault system, the initiation of the right-lateral Las Vegas Valley shear zone in the western Lake Mead domain, and subsequent middle to late Miocene deformation. This study focuses on the lower Horse Spring Formation north of the Las Vegas Valley shear zone. To better understand the stratigraphy and deformation, a detailed geologic map (1:10,000 scale) was produced, data from primary and secondary structures were collected, and ash-fall tuff deposits were dated and correlated through ⁴⁰Ar/³⁹Ar geochronology and tephrachronology. The stratigraphy of the Gale Hills records the initial buttressing of the lower Thumb Member of the Horse Spring Formation onto pre-Tertiary topography. Deposition of the lower Thumb Member records a rapid transgression of the basin margin to the north and northwest across the majority of the Gale Hills. This time was period was then followed by a coarsening up interval and progradation of large alluvial fans in the middle to upper Thumb Member. A transition to a marginal clastic lake in the uppermost Thumb Member then abruptly changed to the Bitter Ridge Limestone algal lake.</p><p> Many studies have evaluated the exhumation history of the Gold Butte block in the eastern Lake Mead domain, which forms the footwall of the major South Virgin-White Hills detachment fault and the relationship with the Frenchman Mountain block. This study shows that the Frenchman Mountain block was just south of the Gale Hills during the early to peak stages of detachment faulting from ca. 17-14 Ma. Two new ⁴⁰Ar/³⁹Ar dates (15.35 Ma) from a prominent ash-fall tuff in the Thumb Member in the northern and southern regions of the western Gale Hills is also in the Frenchman Mountain block. In addition, new tephrachronology correlations have tied Proterozoic-clast debris flows in the western Gale Hills to Proterozoic-clast megabreccia deposits in the Frenchman Mountain block, indicating that the two areas were one connected basin during upper Thumb Member time.</p><p> This study suggests that the left-lateral Government Wash and Southern Gale Hills faults are reactivated northeast-striking, west-down normal faults that were in the correct orientation to be major Riedel prime shears (R') to the right-lateral Las Vegas Valley shear zone. Map and facies relationships show that the Thumb Member deposits were faulted locally during deposition at ca. 15.5 Ma, with increased fault activity and sedimentation rates throughout the Thumb Member after 15.35 Ma and before &sim;14.5 Ma.</p><p> An analysis of structures in the western Gale Hills results in a new model of progressive clockwise rotation and faulting along the Las Vegas Valley shear zone that for the first time honors paleomagnetic results and accounts for all major faulting north of the shear zone. The model is primarily based on clockwise vertical-axis block rotation of domains between oblique left-lateral faults that curve progressively toward the Las Vegas Valley shear zone and terminate into major folds or areas of complex deformation. This model predicts that the western Gale Hills began as a north-northeast elongate block that was reduced in length and elongated in an east-west direction from about 20 to 14 km during translation and rotations. Most of this complex faulting occurred from ca. 13.8 to 8 Ma, after deposition of the Bitter Ridge Limestone.</p>

A study of the Holocene avulsion history of the Brazos alluvial valley revealed that two processes, aggradation and valley tilting, were responsible for generating two styles of avulsion, avulsion-by-progradation and avulsion-by-annexation. As aggradation migrated inland, prograding avulsions tracked the locus of aggradation. Concurrently, a nodal avulsion site developed at 55 km inland, creating avulsions by-annexation. Geomorphic evidence suggests movement along a normal listric fault occurred in proximity to the nodal avulsion site. Within the alluvium above the marine-Oxygen isotope Stage 2 onshore incised valley of the Brazos River, the pattern of stacked channels generated by avulsion was mapped to investigate the aggrading response of the Brazos River to sea level rise. The stacked channels within the valley decrease from eight, at 40 km from the coast, to four, at 65 km from the coast, which reflects the diminishing influence of eustacy inland. As aggradation decreased, while the avulsion frequency remained constant, the younger channels became more isolated, in contradiction to previous stacking models. Those models, however, neglected the influence of antecedent topography during aggradation. Vertically, the eight stacked channels within the lower valley are organized into four stratigraphic units that are attributed to changes in the alluvial valley gradient during aggradation, as calculated from the position of backstepping (retrograding) offshore deltas (paleoshorelines) and their correlative (aggradating) onshore floodplain deposits. The style of avulsion and the channel stacking pattern are both understood with respect to realizable subaerial accommodation. Previous subaerial accommodation models emphasized a proportional upward shift in an equilibrium profile during a sea level rise. Yet, an equilibrium profile must be anchored at both ends. The updip elevation of an alluvial valley is controlled by sediment yield and the cumulative aggradation from all earlier episodes of sea level rise, which should exert a limit on the downdip creation of subaerial accommodation. This study, therefore, quantified the differences in the long-term sediment yield of the Brazos and Trinity rivers of east Texas over past sea level cycles, and concludes that the lower sediment yield of the Trinity River has suppressed its equilibrium profile, thereby limiting the present creation of subaerial accommodation.